m<  A 


;,    University  of  Illinois  M 

Library  at  ^ 

Urbana-Champaign-  '&&!$ 


^-rmk^m^ 

'"%*» 
•j^1 

»\x  i  &msmKBiBUt      .      \^j'   v   • 

"3*.  /A" 


m*^  '^ 

^x^h\  x:<i( 


UNIVERSITY  LIBRARY 

UNIVERSITY  OF  ILLINOIS  AT  URBANA-CHAMPAIGN 

The  person  charging  this  material  is  responsible  for  its 
renewal  or  return  to  the  library  on  or  before  the  due 
date.  The  minimum  fee  for  a  lost  item  is  $125.00, 
$300.00  for  bound  journals. 

Theft,  mutilation,  and  underlining  of  books  are  reasons 
for  disciplinary  action  and  may  result  in  dismissal  from 
the  University.  Please  note:  self-stick  notes  may  result 
in  torn  pages  and  lift  some  inks. 
Renew  via  the  Telephone  Center  at  217-333-8400, 
i  846-262-1510  (toll-free)  or  circlib@uiuc.edu. 

Renew  online  by  choosing  the  My  Account  option  at: 
http://www.library.uiuc.edu/catalog/ 


i  I w 


UNIVERSITY  OF  ILLINOIS 


Agricultural  Experiment  Station 


BULLETIN  No.  108 


SPRAYING  APPLES  FOR  THE 
PLUM-CURCULIO 


BY  s.  A.  FORBES 
STATE  ENTOMOLOGIST 


URBANA,  MAY,  1906 


SUMMARY  OF  BULLETIN  No.    108. 

1.  Experiments  undertaken  in  1904  iri  a  Jackson  county  orchard  for  the 
protection   of   apples   against    injuries  by  plum-curculio,  by  spraying  with 
arsenate  of  lead.  Page  267. 

2.  Tables  of  spraying  experiments  and  their  results.  Page  269. 

3.  Four  times  spraying  of  Ben  Davis  apples  with  arsenate  of  lead  at 
intervals  of  ten  days,  beginning  May  6,   increased  bulk  of  crop  by  54  per 
c::~.t  ,  number  of  apples  by  26  per  cent.,  and  average  size  of  fruit  by  21  per 
cent.     Diminished  curculio  injuries  to  the  apple  by  69  per  cent  ,  which  is  the 
measure  of  the  protection  due  to  treatment.  Page  270. 

4.  Grading  apples  from  experimental  plots  and  from  checks,  and  valuing 
according  to  amount  of  each  grade,  shows  above  treatment  increased  total 
value  of  crop  by^  175  per  cent.  Page  271. 

5.  Evidence  that  curculios  enter  orchard  from  adjacent  woodland. 

Page  274. 

6.  Six  times  spraying  of  Benoni  apples  with  arsenate  of  lead,  beginning 
May  6,   increased  bulk  of  crop  by  40  per  cent.,   and  number  of  apples  by  47 
per  cent.  Page  274. 

7.  Six  times  spraying  of  Ben  Davis  apples,  beginning  May  6,  increased 
bulk  of  crop  by  34  per  cent.,  number  of  apples  by  11  per  cent.,  and  size  of  fruit 
by  24  per  cent.     Sixt}T-eight  per  cent,  of  apples  which  would  have  been  in- 
jured were  protected  from  curculio  injury  by  the  treatment.     Total  value  of 
crop  increased  150  per  cent.  Page  275. 

8.  Winesap  and  Ben  Davis  trees  sprayed  eight  times,  beginning  May  6, 
compared  with  Sops  of  Wine  left  unsprayed  as  checks,  indicate  that  80  per 
cent,  of  the  apples  which  would  have  been  injured  by  curculios  had  been  pro- 
tected against  injurj^  by  the  treatment.  Page  278. 

9.  Cost  of  each  treatment,  including  materials,  labor,  subsistence,  and 
use  of  team,  was  4  to  5  cents  per  tree,  or  17  cents  for  four  treatments  found 

most  effective.  Page  278. 

/ 

10.  Summary  of  results.  Page  279. 

11.  Analysis  of  peelings  from  apples  sprayed  the  preceding  da3'  with  four 
times  the  usual  strength  of  arsenate  of  lead,  showed  .256  of  a  grain  of  arsenic 
to  a  pound  of  the  peelings,  or  approximately  four  pounds  of  the  peelings  to  a 
grain  of  arsenic.     Other  analyses  reported  with  similar  results.         Page  279. 

12.  Precautions  necessary  in  la}ring  out  experimental  and  check  plots, 
and  in  selecting  trees  for  comparison  of  injuries.  Page  281. 

13.  Diagram  of  spraying  effects  with  arsenate  of  lead.  Page  285. 


SPRAYING  APPLES  FOR  THE  PLUM-CURCULIO. 

It  has  been  known  for  more  than  a  hundred  years  that  rVj 
apple  was  subject  to  injury  by  the  plum-curculio,  but  as  this  injury 
has  been  commonly  reckoned  less  important  than  that  done  by  the 
codling-moth,  the  matter  has  received,  of  late,  comparatively  little 
attention  until  within  the  last  six  or  seven  years.  Since  the  dis- 
crimination of  different  market  grades  of  apples  has  been  generally 
introduced — a  grading  based  not  only  upon  size  and  soundness,  but. 
also  upon  superficial  appearance — the  slight  surface  injuries  done 
by  the  new  generation  of  the  plum-curculio,  both  in  laying  the  eggs 
and  in  feeding  on  the  fruit,  have  acquired  a  great  importance.  The 
consequence  of  a  noticeable  injury  of  this  sort,  although  affecting 
but  little,  and  in  many  cases  scarcely  at  all,  the  edible  quality  of 
the  apple  so  injured,  is  to  reduce  its  selling  value  from  No.  i  grade 
to  No.  2,  the  actual  profits  of  the  crop  disappearing  in  this  process. 
These  facts  having  been  repeatedly  brought  to  my  knowledge  by 
Illinois  horticulturists,  I  undertook  to  work  out  the  causes  of  these 
surface  blemishes  in  general,  and  to  demonstrate  more  accurately 
than  has  hitherto  been  done  means  of  preventing  them  by  a  de- 
struction of  the  curculios  themselves.  Indeed,  the  importance  of 
this  investigation  has  grown  upon  me  as  it  has  become  evident  that 
these  small  superficial  injuries  very  often  give  entrance  through  the 
skin  of  the  apple  to  bacteria  and  to  fungus  spores  of  various  kinds, 
causing  rots  and  blights  of  the  fruit  which  might  be  largely  pre- 
vented by  a  timely  destruction  of  the  insects. 

SOME  EARLIER  EXPERIMENTS. 

The  first  exact  experiments  with  insecticides  for  the  protection 
of  apples  against  the  plum-curculio  were  those  made  at  my  office  in 
1885,  on  a  small  orchard  hired  for  the  purpose  near  Urbana,  111.,  the 
results  of  which  were  published  in  one  of  my  reports*,  and  in  the 
Transactions  of  the  Illinois  State  Horticultural  Society  for  that  year. 
In  this  experiment,  which  was  made  primarily  to  test  insecticide 
measures  for  the  destruction  of  the  codling-moth,  the  effect  on  cur- 

*Misc.  Essays  on  Econom.  Ent.,  1886,  pp.  26-45. 


266  BULLETIN  No.  108.  [May, 

culio  injuries  was  likewise  ascertained.  Eight  trees  were  used,  4 
of  them  being  sprayed  and  4  reserved  as  checks.  Two  of  the  trees 
were  treated  with  Paris  green,  I  with  London  purple,  and  I  with 
lime.  All  applications  were  made  eight  times,  beginning  the  gth  of 
June  and  continuing  until  September  3. 

All  the  apples  borne  by  these  trees,  16,529  in  number,  both  those 
which  fell  and  those  which  remained  on  the  tree,  were  examined 
one  by  one,  and  the  results  were  tabulated  for  comparison.  Al- 
though this  was  my  first  work  with  orchard  insecticides,  and  the 
application  was  doubtless  not  made  as  thoroughly  as  would  now  be 
done,  the  general  result  of  the  work  with  Paris  green  was  to  save 
uninjured  on  the  trees  56  per  cent,  of  the  apples  which  would 
otherwise  have  been  injured  by  curculios — a  percentage  less  than 
was  to  have  been  expected  in  orchard  practice  because,  as  noted  at 
the  time,  there  were  other  bearing  trees  in  the  orchard,  from  which 
those  treated  unquestionably  became  reinfested  to  some  extent. 
As  apples  were  not  graded  at  that  time,  no  attempt  was  made  to 
distinguish  differences  in  the  crop  borne  by  trees  sprayed  and  those 
left  unsprayed  as  checks,  beyond  the  mere  number  of  the  injured 
and  uninjured  apples;  but  results  which  I  shall  presently  describe 
make  it  certain  that  the  actual  percentage  of  benefit  would  have  been 
much  greater  than  that  indicated  by  these  figures  if  I  had  also  taken 
account  of  differences  in  size  and  quality  of  the  fruit. 

Because  of  repeated  reports  of  general  and  important,  though 
slight,  surface  injuries  to  apples  by  insects  in  summer  and  fall,  I 
sent  an  assistant,  Mr.  E.  S.  G.  Titus,  through  southern  Illinois  in 
June  and  July,  1901,  to  observe  and  collect  examples  of  these  in- 
juries, and  such  insects  as  might  be  held  responsible  for  them. 
Quantities  of  blemished  fruit  were  sent  to  the  office  for  careful 
study,  and  as  a  means  of  rearing  any  insects  which  it  might  contain. 

From  this  investigation  it  appeared  that  much  the  greater  part 
of  the  injury  complained  of  was  due  to  the  plum-curculio,  and  that 
the  control  of  this  insect  would  virtually  protect  the  apple — a  fact 
reported  to  the  Illinois  State  Horticultural  Society  for  that  year, 
and  published  in  its  Transactions  (p.  148). 

•Pursuing  the  subject  further,  I  read  at  the  meeting  of  this  same 
Society  in  1902  an  article*  prepared  by  Mr.  Titus,  the  greater  part 
of  which  was  given  to  the  work  of  the  plum-curculio  in  the  apple 
orchard.  In  addition  to  a  report  of  many  careful  observations,  an 
important  suggestion  was  made  in  this  paper  to  the  effect  that,  since 
injury  to  the  fruit  was  least  in  well-cultivated  orchards,  cultivation 

*"Insects,  other  than  the  Codlincr-moth,  Injurious  to  the  Fruit  of  the  Apple,"  by  E.  S.  G , 
Titus.  Trans.  III.  State  Hort.  Soc.,  Vol.  36,  pp.  15H-102. 


1906. J  SPRAYING  APPLES  FOR  THE  PLTJM-CURCULIO.  267 

at  a  time  when  the  bulk  of  the  curculios  are  in  the  ground  passing 
through  their  transformations  might  have  the  effect  to  diminish 
their  number  by  killing  them  in  the  pupa  state.  For  this  purpose,  it 
was  said,  a  midsummer  plowing  would  be  necessary,  followed  by 
harrowing  to  break  up  the  clods  and  expose  the  pupae  more  thor- 
oughly to  the  weather.  The  prompt  destruction  of  the  fallen  fruit 
and  spraying  of  the  trees  with  Paris  green  were  also  prominently 
mentioned. 

The  following  year,  in  a  paper*  on  "The  Curculio  and  the 
Apple"  presented  to  the  State  Horticultural  Society,  Professor 
C.  S.  Crandall,  of  the  Horticultural  Department  of  the  Illinois  Ex- 
periment Station,  gave  the  results  of  a  season's  work,  experimental 
and  entomological,  in  an  orchard  in  the  western  part  of  the  state. 
This  article  reports  the  writer's  observations,  and  those  of  his  as- 
sistant, Mr.  James  R.  Shinn,  with  respect  to  many  points  of  inter- 
est in  the  life  history  and  habits  of  the  plum-  and  the  apple-curculios, 
their  numbers  in  the  apple  orchard,  and  their  injuries  to  the  apple, 
together  with  the  outcome  of  certain  spraying  experiments  made  as 
a  test  of  the  insecticide  method  of  controlling  their  injuries.  Ento- 
mologists will  be  especially  interested  in  the  precise  details  of  the  life 
history  and  in  minor  particulars  concerning  the  habits  of  these 
species. 

The  spraying  operation  was  reported  as  unsuccessful,  even  thir- 
teen to  sixteen  applications  of  Paris  green,  extending  from  May  15 
to  August  15,  seeming  virtually  without  effect.  As  only  120  trees 
were  used  in  these  experiments  out  of  an  orchard  of  one  hundred 
and  twenty-five  acres,  and  as  these  experimental  trees  were  divided 
into  plats  of  10  trees  each,  it  is  evident  that  the  sprayed  trees  were 
subject  to  continued  invasion  from  the  surrounding  orchard,  suffi- 
cient, possibly,  to  obliterate  the  results  of  the  treatment.  A  sugges- 
tion which  I  have  already  mentioned — that  of  plowing  and  harrow- 
ing infested  orchards  in  midsummer  to  destroy  the  pupae  in  the 
earth — is  here  reinforced  with  numerous  exact  observations  as  to 
the  time  of  pupation,  and  the  depth  to  which  the  curculio  larvae  pen- 
etrate for  their  pupal  transformations. 

EXPERIMENTS  OF  1904. 

Owing  to  the  unfavorable  outcome  of  these  experiments  of  1903 
for  the  control  of  curculio  injuries  by  the  use  of  insecticides,  it  be- 
came important  that  measures  should  be  taken  to  test  anew  the  use- 
fulness of  the  arsenical  sprays,  especially  as  these  are  necessary 
to  any  program  of  fairly  complete  protection  to  the  crop  of  apples  on 

*Trans.  111.  State  Hort.  Society,  Vol.  37,  pp.  176-189. 


268 


BULLETIN  No.  108. 


[Mai/, 


the  trees.  Destruction  of  windfalls  and  midsummer  plowing  have 
the  disadvantage  that  they  lock  the  stable  after  one  of  the  horses,  at 
least,  is  stolen,  since  they  can  at  most  prevent  only  injuries  by  the 
new  brood  of  curculios  appearing  after  midsummer.  In  the  ab- 
sence of  poison  sprays,  an  orchard  freed  of  curculios  in  midsummer 
—if  this  is  possible — by  plowing  and  harrowing,  would  be  liable  to 
invasion  and  serious  injury  later  in  the  season  by  insects  entering 
it  from  outside,  and  would  also  be  exposed  to  injuries  in  the  spring 
and  early  summer  of  the  following  year.  With  these  facts  in  mind, 
I  undertook,  in  1904,  a  field  experiment  in  southern  Illinois  intended 
to  test  the  practical  value  of  an  arsenical  poison  directed  immedi- 
ately against  the  plum-curculio,  with  the  understanding  that  similar 
experiments  were  to  be  carried  on  by  the  Horticultural  Department 
of  the  State  Experiment  Station  in  another  part  of  the  state.  My 
experiment  was  placed  in  charge  of  my  field  assistant,  Mr.  E.  P. 
Taylor,  and  this  report  is  based  on  his  field  notes  and  specimens. 

The  farm  selected,  after  an  examination  of  some  twenty-five 
fruit  farms  in  southern  Illinois,  contained  three  small  orchards 
aggregating  eighteen  acres.  It  was  situated  two  miles  south  of 
Carbondale,  in  Jackson  county.  These  orchards  were  chosen  partly 
because  of  the  considerable  crop  which  they  bore — an  unusual  thing 
that  year — and  also  because  the  separation  of  the  varieties  with 
which  they  were  planted,  enabled  us  to  make,  in  most  cases,  check 
and  experimental  plots  of  the  same  variety. 


Woods 


Pasture 


ORCHARD 


NO.l 


Wheat 


ORCH'D!  NO. 


Meadov 


Pasture 


ORCHARD. 


NO.  3 


Oats 


Meadov 


SITE   AMD   SURROUNDINGS   OF  EXPERIMENTAL    ORCHARDS 


1906.] 


SPRAYING  APPLES  FOR  THE  PLUM-CURCULIO. 


269 


The  insecticide  used  was  a  compound  of  arsenate  of  soda  and 
acetate  of  lead  commonly  known  as  the  arsenate  of  lead.  It  was 
made  by  dissolving  12^  ounces  of  the  acetate  of  lead  and  5  ounces 
of  arsenate  of  soda  in  separate  vessels,  and  pouring  the  two  solu- 
tions, one  after  the  other,  into  a  barrel  with  50  gallons  of  water. 
Two  barrels  were  placed  in  the  wagon  at  once,  each  with  a  No.  2 
Morrill  and  Morley  spray  pump  carrying  two  lines  of  hose  about 
twenty-five  feet. long,  furnished  with  twelve-foot  extension  poles 
fitted  with  double  Vermorel  nozzles. 

In  spraying,  as  high  a  pressure  was  used  as  could  be  kept  up  by 
hand ;  and  a  fine  mist  of  the  spray  was  forced  into  the  trees  from 
every  side,  care  being  taken  to  wet  thoroughly  all  parts  of  the  foliage 
without  causing  the  drops  to  run  in  streams  upon  the  leaves.  The 
varieties  of  apples  in  the  orchard  will  be  referred  to  in  giving  tihe 
results  of  the  experiments. 

Three  sets  of  experiments  were  made  by  spraying  four,  six, 
and  eight  times  respectively,  at  intervals  of  about  ten  days,  begin- 
ning May  6  to  10,  and  ending  July  28.  At  the  time  of  the  first 
spraying  at  Carbondale,  the  apple-trees  were  in  their  first  full  bloom. 

RESULTS  OF  EXPERIMENTS. 

As  a  basis  for  the  discussion  of  the  principal  results  of  this 
experiment,  I  have  prepared  the  accompanying  table  showing  espe- 
cially the  differences  in  quantity,  number,  and  quality  of  apples  borne 
by  trees  sprayed  and  by  those  without  treatment.  This  table,  it 
will  be  noticed,  is  divided  into  two  parts,  an  upper  and  a  lower,  and 
for  convenience  in  reference  I  have  numbered  the  vertical  columns 
from  i  to  12  and  the  horizontal  sections  of  the  table  from  i  to  8. 

CURCULIO  SPRAYING  EXPERIMENTS. 


00 

C 

.2 

"o 
o> 

30 

1 

2 

Columns 

1 

2 

3 

4 

5 

6 

Plot 

Times 
spray- 
ed 

Trees  used 

Apples  picked 

Injured 
per  cent. 

No. 

C't'd 

Date 

Bushels 

Number 

[,  Check 
Exp'er. 

0 
4 

115 
111 

4 
4 

S.  16-20 
S.  16-20 

15.5 
23.9 

2,617 
3,308 

92.2 
28.59 

II,  Check 
Exper. 

0 
6 

34 
32 

3 
3 

Jl.  26-29 
Jl.  26-29 

11.4 
16. 

2,593 

3,782 

21.4 
103 

3 

Ill,  Check 
Exper. 

0 
6 

20 
20 

3 
3 

S.  17 
S.  17 

16.25 
22. 

2,770 
3,063 

95.6 
30.7 

4 

IV,  Check 
Exper. 

0 

8 

145 
119 

(9)4 
4 

Jl.  15 
S.  16-20 

6.44 

24.44 

1,540 
3,198 

91.1 
185 

270  BULLETIN  No.  108.  [May, 

CURCULIO  SPRAYING  EXPERIMENTS. — Continued. 


Sections  1 

Columns 

7 

8 

9 

10 

11 

12 

Plot 

Injuries 
prevented 
percent. 

Yield 
increased 
per  cent. 

Bushels 

Value 
increased 
times 

No.  1 

No.  2 

No.  8 

5 
6 

I,  Check 
Exper. 

69. 

54. 

1.5 
17.16 

6.2 
5.5 

7.8 
1.16 

2.75 

II,  Check 
Exper. 

48. 

40. 

7 

Ill,  Check 
Exper. 

68. 

34. 

.75 
12.06 

3.75 
3.25 

11.66 
6.66 

2.5 

8 

IV,  Check 
Exper. 

80. 

Orchard  I  (Plot  /.). — By  an  examination  of  this  table  it  may  be 
seen  (Section  i,  Columns  i  to  5)  that  all  the  apples  on  4  trees 
of  Plot  I.  out  of  in  of  the  Ben  Davis  variety  which  had  been 
sprayed  four  times  with  arsenate  of  lead,  were  picked,  measured, 
counted,  and  carefully  examined  between  the  i6th  and  the  2Oth  of 
September,  and  that  all  the  apples  on  4  other  trees  of  this  plot  out  of 
115  Ben  Davises  which  had  been  left  unsprayed  as  a  check,  were 
also  picked,  measured,  counted,  and  similarly  examined  at  the  same 
time.  These  two  lots  of  4  trees  each  had  been  so  selected  at  the 
beginning  of  the  experiment  from  the  whole  number  in  each  lot  as 
to  make  the  check  lot  the  nearest  possible  duplicate  of  the  lot  which 
had  been  sprayed,  except,  of  course,  in  the  mere  matter  of  spraying*. 


SPRAVED  -4  TIMES 


NOT  SPRAYED 


A 

•   4-               •    •                            .    *    . 

i 

i 

+   •   i 

[•••*•••     •    ^    •    ^   A    •    -A-   •    -^    •    -^   •    ^    •    ^ 

i 

•        * 

ORCHARD     NOI.       APPL-ES    COUNTED     FROM    STARRETD    TREES 

*The  four  sprayed  trees  used  for  comparison  with  the  four  check  trees  are  those  starred  in 
the  sixth  row  from  the  middle  line  of  the  orchard.  The  four  trees  used  as  checks  are  among1 
those  starred  in  the  fifth  and  sixth  rows  from  the  middle  line. 


1906.]  SPRAYING  APPLES  FOR  THE  PLUM-CURCULIO.  271 

The  4  trees  not  sprayed  yielded  15.5  bushels  of  apples,  and  the  4 
trees  sprayed  yielded  23.9  bushels  (Section  i,  Column  4),  the  bulk 
of  the  yield  being  thus  54  per  cent,  greater  for  the  sprayed  trees  than 
for  those  unsprayed  (Section  5,  Column  8).  In  other  words  the 
trees  which  had  been  treated,  yielded  one  and  a  half  times  as  large  a 
quantity  of  apples  as  those  which  had  not  been  sprayed. 

The  number  of  apples  from  the  4  trees  not  treated  (Column  5) 
was  2,617,  while  that  from  the  trees  treated  was  3,308 — a  difference 
of  26  per  cent,  in  favor  of  the  sprayed  trees.  That  is,  the  sprayed 
trees  bore  a  fourth  -more  apples  than  those  which  had  not  been 
sprayed.  We  should  notice,  in  passing,  that  the  apples  from  the 
trees  not  treated  ran  169  to  the  bushel,  and  that  those  from  the 
treated  trees  ran  139  to  the  bushel — a  difference  of  21  per  cent,  in 
average  size  of  fruit  in  favor  of  the  sprayed  trees.  That  is,  the  ap- 
ples on  the  trees  which  had  been  sprayed  were  one  fifth  larger,  on  an 
average,  than  were  those  from  the  other  trees. 

All  these  differences  were  apparent  without  any  special  examina- 
tion of  the  fruit  with  reference  to  injuries  by  curculios.  When  these 
curculio  injuries  were  distinguished  and  tabulated  (Column  6),  it 
was  found  that  92.2  per  cent,  of  the  apples  on  the  unsprayed  trees 
had  been  injured  by  curculios  making  feeding-pits,  egg  punctures,  or 
both,  and  that  28.6  per  cent,  of'  the  apples  on  the  sprayed  trees  were 
so  injured.  A  simple  calculation  from  these  data  shows  (Section  5, 
Column  7)  that  69  per  cent,  of  the  apples  which  would  have  been  in- 
jured if  no  treatment  had  been  applied,  had  been  protected  from 
injury  by  the  arsenate  spray. 

Next,  the  total  product  of  both  lots  of  trees  was  separately  and 
very  carefully  graded  as  No.  I's,  2's,  and  3's,  by  the  standards  of  the 
American  Apple  Growers'  Association  adopted  in  1903,  (Section  5, 
Columns  9,  10,  and  n,)  with  the  result  to  show  that  the  4  check 
trees  yielded  il/2  bushels  of  No.  I's,  and  the  4  sprayed  trees,  17.16 
bushels;  that  the  check  trees  yielded  6.2  bushels  of  No.  2's,  and  the 
sprayed  trees,  5^  bushels;  that  7.8  bushels  of  No.  3's  were  borne 
by  the  check  trees,  and  1.16  bushels  by  the  trees  which  had  been 
sprayed.  (See  Plates  I.  and  II.) 

According  to  the  best  estimate  we  can  make  of  the  market  values 
of  these  three  grades,  if  the  No.  i  apples  sell  for  $i,  No.  2's  might 
be  expected  to  bring  75  cents,  and  No.  3*3,  25  cents  or  thereabouts. 
Applying  these  estimates  to  these  various  lots  of  apples,  it  appears 
that  the  actual  value  of  the  crop  from  the  treated  trees  was  2^4 
times  that  from  the  trees  which  had  not  been  treated  (Column  12). 


272 


BULLETIN  No.  108. 


[May, 


1906.] 


SPRAYING  APPLES  FOR  THE  PLUM-CURCULIO. 


273 


274  BULLETIN  No.  108.  [May, 

To  see  whether  the  outer  rows  of  trees  were  any  more  likely  to 
be  heavily  infested  by  curculios  than  the  inner, — whether  curculios 
entering  the  orchard  from  outside  would  accumulate  upon  the  outer 
rows, — three  trees  in  the  sprayed  lot  were  examined,  one  from  the 
middle  and  one  from  each  end  of  the  row  farthest  removed  from 
the  check.  No  differences  in  amount  of  curculio  injury  were  found 
in  these  trees,  as  compared  with  the  average  for  the  plot  in  general, 
with  the  exception  of  one  of  the  three,  which  stood  at  a  corner  of 
the  orchard  next  a  fringe  of  forest  trees  and  brush.  The  fruit  of 
this  tree,  notwithstanding  the  four  thorough  sprayings  with  arsenate 
of  lead  which  it  had  received,  had  been  injured  by  curculios  to  the 
amount  of  nearly  87  per  cent.,  showing  an  extensive  immigration 
into  the  orchard  from  the  adjoining  woodland.  As  this  contained 
both  hawthorn  and  wild  cherry-trees  it  doubtless  furnished  a  con- 
stant breeding  ground  for  curculios. 

Foreseeing  difficulties  of  this  kind,  and  also  the  probability  of 
a  mutual  influence  of  check  and  experimental  plots,  the  precaution 
had  been  taken  to  select,  quite  early  in  the  season,  for  comparison, 
four  trees  from  the  center  of  each  plot,  and  it  is  thus  quite  certain 
that  no  outside  interference  or  intermingling  of  effects  influenced 
the  contrast  obtained  in  this  experiment. 

Orchard  2  (Plots  II.  and  III.}. — We  turn  next  to  the  second  or- 
chard (Sections  2,  3,  6,  and  7),  one  half  of  which  was  sprayed  six 
times  in  succession,  the  other  half  being  left  as  a  check.  As  a  part 
of  these  apples  were  of  an  early  variety  (Benoni),  and  a  part  were 
of  a  late  variety  (Ben  Davis),  I  will  give  the  results  for  these  two 
varieties  separately. 

Thirty-two  Benoni  trees  (Plot  II.)  were  sprayed  six  times  in 
this  experiment,  in  comparison  with  34  Benoni  trees  left  as  a  check, 
and  the  apples  borne  by  3  trees  from  each  of  these  two  lots  were 
picked,  measured,  counted,  and  examined  July  26  to  29  (Section  2, 
Columns  I  to  5),  the  two  sets  of  trees  having  been  so  selected  as 
to  make  the  check  lot  the  nearest  possible  duplicate  of  those  which 
had  been  sprayed.  The  3  trees  not  sprayed  yielded  11.4  bushels  of 
apples,  and  those  which  had  been  sprayed  yielded  16  bushels  (Col- 
umn 4) — an  increase  of  40  per  cent,  in  the  bulk  of  the  yield  as  a 
consequence  of  the  spraying.  The  3  trees  which  had  not  been  treated 
bore  2,593  apples,  while  the  3  treated  trees  bore  3,782  (Column  5) 
— a  difference  of  47  per  cent,  in  favor  of  the  treated  trees.  That  is, 
the  sprayed  trees  bore  nearly  half  as  many  apples  again  as  those 
whicli  had  not  been  sprayed.  The  average  size  of  the  apples  was 
approximately  the  same  for  both  lots — 228  per  bushel  for  the  un- 
treated trees,  and  236  for  the  treated. 


1906.]  SPRAYING  APPLES  FOR  THE  PLUM-CURCULIO.  275 

Turning  next  to  the  ratios  of  curculio  injury  to  these  Benoni 
apples  (Column  6),  we  find  that  21.4  per  cent,  of  the  fruit  of  the  un- 
sprayed  trees  had  been  injured  by  curculios,  and  10.3  per  cent,  of 
that  on  the  sprayed  trees,  48  per  cent,  of  the  apples  which  would  have 
been  injured  having  been  protected  from  injury  by  the  arsenical 
spray.  These  Benoni  apples  were  harvested  in  July,  but  were  not 
graded  into  market  classes.  I  consequently  have  no  data  as  to  the 
total  money  benefit  of  the  spraying  shown  by  this  Plot  II. 

On  the  other  part  of  this  orchard  (Plot  III.),  sprayed  six  times 
( Sections  3  and  7) ,  were  20  Ben  Davis  trees  which  are  to  be  compared 
with  an  equal  number  of  this  variety  left  unsprayed  as  a  check. 
From  each  of  these  lots  again  3  trees  were  taken  for  critical  com- 
parison of  the  product  of  those  sprayed  with  that  of  those  unsprayed 
(Section  3,  Columns  i,  2,  and  3).  The  final  count  on  all  these  trees 
was  made  September  17.  The  3  trees  not  sprayed  gave  16%.  bushels 
of  apples,  while  the  3  trees  sprayed  yielded  22  bushels  (Column  4) 
— an  increase  of  34  per  cent,  in  quantity  of  apples  as  a  consequence 
of  the  spraying.  The  3  Ben  Davis  trees  which  had  not  been  treated 
bore  2,770  apples,  while  the  3  treated  trees  gave  3,063  (Column  5) 
— a  difference  of  only  n  per  cent,  in  favor  of  the  treated  trees. 
The  unsprayed  apples  on  these  trees  ran.  i/o^to  the  bushel,  and  the 
sprayed  apples,  139  to  the  bushel — a  difference  of  24  per  cent,  in 
size  of  fruit  in  favor  of  the  trees  which  had  been  sprayed. 

With  respect  to  the  curculio  injury  to  these  Ben  Davis  apples 
we  find  that  95.6  per  cent,  of  the  unsprayed  fruit  had  been  injured 
by  curculios,  and  30.7  per  cent,  of  that  sprayed ;  or,  in  other  words, 
that  68  per  cent,  of  the  apples  which  would  have  been  injured  had 
been  protected  from  injury  by  the  treatment  received. 

Looking  now  to  the  quality  of  the  fruit  from  these  Ben  Davis 
trees,  which  had  been  sprayed  six  times  in  all  (Section  7,  Columns  9. 
10,  and  n),  we  find  that  the  3  unsprayed  trees  examined  gave  a 
yield  of  %  of  a  bushel  of  No.  I  apples,  while  the  3  sprayed  trees 
yielded  12.06  bushels;  that  the  unsprayed  trees  gave  3^4  bushels  of 
No.  2's,  the  sprayed  trees  3  *4  bushels ;  that  the  unsprayed  trees  yielded 
ii^j  bushels  of  No.  3's,  and  the  sprayed  trees  62/$  bushels.  (See 
Plates  III.  and  IV.)  Reducing  these  quantities  to  comparable 
money  values  by  the  prices  already  assigned,  we  find  that  the  actual 
value  of  the  crop  from  the  treated  trees  was  2l/2  times  that  from  the 
trees  which  had  not  been  treated. 

Orchard  3  (Plot  IV.}. — In  the  third  and  last  orchard  on  which 
I  have  to  report,  the  check  and  the  experimental  sections  do  not  cor- 
respond closely,  the  sprayed  trees  being  all  Winesap  and  Ben  Davis, 
late  varieties  only,  and  the  check  lot  almost  entirely  of  early  vari- 


276 


BULLETIN  No.  108. 


[May, 


1906,]  SPRAYING  APPLES  FOR  THE  PLUM-CURCULIO. 


277 


278  BULLETIN  No.  108.  [May, 

eties.  As  the  early  varieties  were  harvested  July  15,  they  were  less 
exposed  to  injury  than  the  fruit  on  the  sprayed  trees,  which  was 
harvested  from  September  16  to  20.  This  is  a  difference,  however, 
which  tends  to  diminish  the  contrast  between  the  sprayed  and  un- 
sprayed  trees,  and  thus  to  show,  if  anything,  a  smaller  result  from 
spraying  than  was  actually  obtained. 

The  Winesap  and  Ben  Davis  trees,  119  in  number,  were  sprayed 
eight  times  in  succession,  and  the  fruit  from  4  of  these  trees  (3  Ben 
Davis  and  I  Winesap)  was  picked  and  examined  with  reference  to 
curculio  injuries.  The  unsprayed  orchard  used  as  a  check  on  this 
experiment  contained  145  trees,  all  Sops  of  Wine  and  other  early 
varieties  except  4  Ben  Davis  (see  Section  4,  Columns  i,  2,  and  3). 
The  fruit  from  9  of  these  check  trees  was  examined  July  15,  all  Sops 
of  Wine  except  i  Ben  Davis.  These  yielded  3,463  apples,  making 
14%  bushels,  equal  to  1,540  apples,  or  6.44  bushels  from  4  trees 
(Columns  4  and  5),  while  the  4  trees  sprayed,  the  apples  from  which 
were  picked  September  16  to  20,  yielded  3,198  apples,  making 
24^/2  bushels.  These  differences  in  number  of  apples  and  bulk  of 
yield  can  not  properly  be  taken  into  account  in  this  case,  because 
both  might  well  be  attributable,  at  least  in  part,  to  the  difference  in 
variety  between  the  sprayed  and  the  check  trees  from  which  the 
fruit  was  counted. 

The  curculio  injuries  on  the  9  unsprayed  trees  had  affected  an 
average  of  91  per  cent,  of  the  apples  (Column  6),  and  on  the  4 
sprayed  trees,  an  average  of  18^  per  cent.,  from  which  it  appears 
(Section  8,  Column  7)  that  80  per  cent,  of  the  apples  which  might 
have  been  injured  by  curculios  if  no  treatment  had  been  applied, 
had  been  saved  from  such  injury  by  the  arsenical  spray.  Here  again 
it  is  possible  that  a  varietal  difference  in  susceptibility  to  curculio 
injury  may  have  affected  these  percentages. 

COST  OF  THE  OPERATION. 

The  reader  will  be  especially  interested  to  know  the  cost  of  our 
operation,  figured  in  terms  practical  to  the  fruit  grower.  I  have 
worked  this  out  in  full  detail  for  the  292  trees  sprayed  in  these  or- 
chards, including  the  actual  price  of  materials  purchased  in  the  ordi- 
nary markets,  the  amount  of  materials  used,  and  the  labor  of  applica- 
tion at  $1.25  a  day  and  board  for  men  employed,  and  $2.50  a  day  and 
feed  for  a  team.  Without  entering  into  details,  unnecessary  for  my 
purpose,  I  find  that  the  total  cost  ranges  from  4  to  5  cents  per  tree 
for  each  treatment,  or  17  cents  per  tree  for  the  four  treatments 
found  most  effective.  Of  this  only  2  cents  was  for  materials,  the 
remaining  1 5  cents  being  for  labor  of  man  and  beast,  much  of  which, 
in  many  cases,  the  fruit  grower  might  supply  without  special  outlay. 


1906.]  SPRAYING  APPLES  FOR  THE  PLUM-CURCULIO.  279 

SUMMARY  OF  SPRAYING  RESULTS. 

Finally,  to  sum  up  in  a  word  the  most  important  practical  results 
of  the  orchard  experiment  with  arsenate  of  lead,  we  may  say  that 
four  sprayings  of  apple-trees  of  late  varieties,  exposed  to  a  very 
heavy  attack  by  the  plum-curculio,  the  first  spraying  applied  in  early 
May  just  as  the  trees  are  coming  into  bloom,  and  the  others  at  in- 
tervals of  10  days  thereafter,  the  whole  operation  costing  17  cents 
per  tree,  may  be  expected  to  increase  the  yield  of  the  orchard  about 
one  half,  to  increr.se  the  average  size  of  the  fruit  by  about 
a  fifth,  and  so  to  improve  the  quality  of  the  apples  that  they  should 
be  worth  from  2^2  to  3  times  as  much  as  if  the  orchard  had  not  been 
sprayed. 

POISON  TESTS  OF  SPRAYED  APPLES. 

The  arsenate  of  lead  used  in  experiments  described  in  this  paper 
is  an  unusually  adhesive  insecticide,  remaining  upon  trees  in  visible 
quantity  weeks  and  even  months  after  it  has  been  applied.  Evident 
traces  of  the  mixture  were  still  visible  in  October  on  leaves  which 
had  fallen  from  trees  sprayed  with  the  arsenate  of  lead  July  4.  In 
view  of  this  fact  it  seemed  to  me  important  that  experiments  should 
be  made  to  determine  the  amount  of  poison  carried  by  apples  treated 
with  this  insecticide  within  a  reasonable  limit  of  time  after  the  spray 
was  applied.  I  consequently  directed,  in  1905,  the  spraying  of  apple- 
trees  with  various  arsenical  compounds,  and  among  others  with  the 
home-made  arsenate  of  lead,  and  obtained  analyses  of  the  peelings 
of  apples  so  sprayed  taken  from  2  trees  within  a  day  of  the  applica- 
tion of  the  insecticide,  and  from  a  third  tree  2  months  thereafter. 

The  first  of  these  trees,  of  the  Duchess  variety,  received  2  spray- 
ings with  the  arsenate  of  lead — the  first  June  9  and  the  second  July  4 
— not  having  been  previously  sprayed  at  all  that  year.  In  both 
these  sprayings  the  insecticide  was  used  at  4  times  the  common 
strength — 12^2  ounces  of  acetate  of  lead  and  5  ounces  of  arsenate 
of  soda  to  i2l/2  gallons  of  water  instead  of  the  usual  50  gallons. 

The  next  day,  July  5,  six  apples,  averaging  two  and  a  fourth  inches 
in  diameter,  were  carefully  picked  from  this  tree  by  the  stem  and  peeled 
at  once,  the  apple  being  held  upon  the  point  of  one  knife  while  it 
was  peeled  with  another,  to  avoid  removing  any  part  of  the  poison 
unnecessarily.  The  peelings  were  then  dropped  into  a  dry,  clean, 
new  Mason  jar  and  submitted  for  chemical  analysis  to  Dr.  Wm. 
M.  Dehn,  of  the  Chemical  Department  of  the  University  of  Illinois. 
It  should  be  said  that  the  weather  in  the  interval  between  spraying 
and  collection  of  the  apples  had  been  dry.  According  to  Dr.  Dehn's 


280  BULLETIN  No.  108.  [May, 

report,  these  apple  peelings  thus  collected  yielded  36.6  parts  per  mil- 
lion of  arsenious  acid,  enough  to  equal  .2562  grains  of  arsenic  to  an 
avoirdupois  pound  of  the  sample  peelings.  This  would  mean  that 
one  would  have  to  eat  approximately  four  pounds  of  apple  peelings 
to  get  a  grain  of  arsenic  if  the  fruit  were  taken  the  day  after  spray- 
ing from  a  tree  which  had  received  four  times  the  usual  strength  of 
this  insecticide. 

Another  tree,  a  Benoni,  differed  especially  in  two  respects  from 
the  foregoing.  It  had  been  sprayed  three  times  by  the  owner  earlier 
in  the  season,  and  it  was  sprayed  twice  by  us — June  9  and  July  4 — 
but  with  arsenate  of  lead  in  the  strength  usual  in  practical  orchard 
work,  that  is,  i2l/2  ounces  of  acetate  of  lead  and  5  ounces  of  arsenate 
of  soda  to  50  gallons  of  water.  The  earlier  sprayings  were  as  fol- 
lows :  First  spraying  about  April  3,  before  the  appearance  of  the 
bloom,  with  3  pounds  copper  sulphate,  5  to  6  pounds  lime,  2  ounces 
arsenate  of  soda,  and  2  ounces  Paris  green,  to  50  gallons  of  water ; 
second  spraying  about  April  26,  just  after  the  bloom  had  fallen,  with 
2  pounds  copper  sulphate,  i  pound  Lorenberg's  arsenate  of  lead, 
2  ounces  Paris  green,  3  to  4  pounds  lime,  and  50  gallons  of 
water.  Rain  fell  more  or  less  for  three  days  after  this  second  spray- 
ing. The  third  spraying,  about  May  3,  was  with  2  pounds  copper 
sulphate,  3  to  4  pounds  lime,  i  pound  Lorenberg's  arsenate  of  lead, 
2  ounces  Paris  green,  and  50  gallons  of  water. 

From  this  tree  six  apples  averaging  two  inches  in  diameter 
were  picked  July  5,  the  day  after  our  own  second  spraying,  with  the 
precautions  described  above.  The  peelings  tested  by  Dr.  Dehn 
yielded  32.9  parts  of  arsenious  acid  per  million,  equivalent  to  .2303 
grains  of  arsenic  per  pound  of  the  samples. 

It  will  be  noticed  that,  although  the  insecticide  here  used  con- 
tained only  one  fourth  as  much  of  the  arsenate  as  that  applied  to  the 
Duchess  tree,  the  percentage  of  arsenic  remaining  on  the  fruit  was 
nearly  as  large. 

The  third  tree  treated  was  a  Rome  Beauty,  sprayed,  like  the 
preceding,  three  times,  April  3,  April  26,  and  May  3,  approximately, 
with  combinations  of  Bordeaux  mixture  and  arsenical  insecticides, 
and  further  sprayed,  June  9  and  July  4,  with  home-made  arsenate  of 
lead  at  the  rate  of  12^ — 5 — 12^2  ;  that  is,  like  the  first  tree,  with 
four  times  the  usual  strength. 

The  apples  were  picked  from  this  tree  September  4,  two  full  months 
from  the  time  of  spraying.  Those  chosen  showed  more  than  the  aver- 
age amount  of  residue  of  the  insecticide  on  the  fruit.  They  were 
picked  into  baskets,  poured  into  a  box  where  they  were  packed  closely 
by  hand,  and  shipped  to  the  office  at  Urbana,  being  given  average 


woe.]  SPRAYING  APPLES  FOR  THE  PLUM-CURCULIO.  281 

shipping  treatment.  Ten  apples  were  later  taken  from  the  box  and 
carefully  peeled,  the  peelings  being  submitted,  as  before,  to  Dr.  Dehn 
for  examination.  Notwithstanding  the  long  exposure  in  the  or- 
chard, the  report  on  the  arsenical  contents  of  this  material  is  very 
similar  to  that  of  the  preceding  determinations.  <The  arsenious  acid 
was  found  at  the  rate  of  40.1  parts  per  million,  enough  to  give  .2807 
grains  per  pound  of  the  apple  peelings.  One  would  thus  have  to  eat 
about  three  and  a  half  pounds  of  these  peelings  to  get  a  grain  of 
arsenic. 

Two  grains  of  arsenic  is  a  fatal  dose,  and  one  thirtieth  to  one 
tenth  of  a  grain  is  a  medicinal  dose.  For  the  first,  one  would  have  to 
eat  seven  or  eight  pounds  of  these  apple  peelings ;  and  for  the  sec- 
ond, from  two  to  seven  ounces.  While  these  facts  are  not  at  all 
alarming,  they  suggest  discretion  in  the  use  of  this  insecticide  and 
in  the  subsequent  handling  and  disposal  of  the  apples.  We  also 
need  further  experiment  with  various  strengths  of  the  spray  and 
with  various  kinds  and  amounts  of  subsequent  exposure  of  the  fruit. 
Nevertheless,  in  view  of  the  fact  that  apple  peelings  are  rarely  eaten 
in  any  quantity,  I  think  that  no  hesitation  need  be  felt  to  substitute 
the  arsenate  of  lead  for  the  more  usual  insecticide  sprays  if  the 
facts  here  given  are  borne  in  mind  and  duly  allowed  for. 

THE  NEUTRAL  ZONE  IN  SPRAYING  EXPERIMENTS. 

It  has  been  customary  in  making  field  insecticide  experiments  to 
apply  the  chosen  treatment  to  a  certain  area,  leaving  a  corresponding 
area  immediately  beside  it  untreated,  as  a  check,  the  utility  of  the 
treatment  being  determined  by  a  comparison  of  subsequent  condi- 
tions on  these  two  tracts.  Attention  has  already  been  called  by  Gil- 
lette,* Weed,f  and  myself,  |  to  the  fact  that  the  results  of  such  ex- 
periments must  be  somewhat  affected  by  the  spread  of  insects  from 
the  so-called  check  plot  to  the  experimental  plot,  but  nothing  has 
been  done  to  demonstrate  this  proposition  experimentally,  or  to 
show  just  how  important  this  mutual  influence  of  check  and  experi- 
mental plots  may  be. 

In  the  spraying  experiment  conducted  in  Orchard  L,  as  above  de- 
scribed in  this  article,  the  opportunity  was  improved  to  ascertain 
to  what  extent  the  curculios  spread  from  one  part  of  the  orchard  to 
another — to  what  extent,  that  is,  one  must  guard  against  vitiation 
of  results  of  spraying  experiments  by  the  spread  of  the  curculios 
from  unsprayed  trees  to  those  which  have  been  sprayed.  This  or- 


*Bull.  M1890),  la.  AST.  Ex£er.  Station,  pp.  383-384. 

'  ericnl.  Science, 

State  Eut.   111.,    p.    9; 


.  . 

tBull.  23  (1890),  Ohio  Agr.  Exper.  Station,  p.  226;  Agricul.  Science,  Apr.,  1890,   p.  97. 
3:"Arsenical  Poisons  for  the  Codling-moth."        Bull.  1  (1887),  Office  ~ 


Fifteenth  Report,  State  Ent.  111.,  p.  13. 


282  BULLETIN  No.  108.  [May, 

chard,  it  will  be  remembered,  was  divided  into  two  equal  parts,  each 
virtually  square,  on  one  of  which  all  trees  were  sprayed  four  times 
with  arsenate  of  lead,  beginning  May  6  and  following  at  intervals  of 
ten  days  thereafter,  while  the  other  part  was  left  unsprayed  as  a 
check.  The  apples  in  this  orchard  were  of  the  Ben  Davis  variety, 
and  were  harvested  September  20.  At  this  time  all  the  apples 
borne  by  three  trees  which  had  not  been  sprayed,  but  which  stood 
in  the  row  next  the  sprayed  half  of  the  plot,  were  counted  and  exam- 
ined in  comparison  with  the  product  of  ten  other  trees  taken  at 
various  points  in  this  unsprayed  part  of  the  orchard.  (See  diagram, 
p.  270,  and  table  on  p.  284).  Eighty-one  and  five  tenths  per  cent, 
of  the  apples  on  these  3  trees  next  the  experimental  plot  had  been 
injured  by  the  curculios,  and  90^3  per  cent,  of  the  apples  on  the  10 
trees  farther  back, — a  difference  of  8.8  per  cent,  of  all  the  apples  on 
these  trees,  which  can  only  be  accounted  for  on  the  supposition  that 
more  curculios  went  from  the  three  trees  nearest  the  sprayed  half  of 
the  orchard  into  that  half  than  came  into  it  from  that  half. 

Next,  three  trees  which  had  been  sprayed  and  which  stood  in  the 
row  nearest  the  unsprayed  part  of  the  orchard  were  similarly  exam- 
ined, with  the  result  that  43^  per  cent,  of  the  apples  on  these  trees 
were  found  to  have  been  injured  by  curculios,  while  26^2  per  cent, 
of  the  apples  on  10  other  trees  taken  elsewhere  in  this  sprayed  plot 
were  so  injured.  That  is,  16.8  per  cent,  more  of  the  fruit  on  the 
trees  standing  nearest  the  unsprayed  portion  was  injured  than  on 
the  trees  farther  back. 

It  will  be  seen  that  the  increase  of  average  injury  to  apples  on 
unsprayed  trees  near  the  check  plot  was  nearly  twice  as  great  as 
the  decrease  of  injury  on  the  unsprayed  trees  standing  next  the 
experimental  plot.  This  can  only  mean  that  the  excess  of  curculios 
on  the  unsprayed  fruit  to  which  their  greater  injury  was  due  did  not 
all  come  from  the  unsprayed  trees  in  the  adjoining  row,  but  that 
approximately  half  of  them  must  have  come  in  from  remoter  parts 
of  the  experimental  plot.  In  order  to  see  how  far  this  mutual  effect  of 
one  plot  on  the  other  actually  extended  inward  in  either  direction 
from  the  dividing  line,  apples  were  picked  and  counted  separately 
for  each  tree  from  a  series  of  14  trees  running  lengthwise  of  the  or- 
chard at  right  angles  to  the  dividing  line  between  the  two  plots.  It 
was  found  in  this  way  that  notable  dimunition  of  curculio  injury  on 
the  one  side  and  increase  on  the  other  did  not  extend  so  far  as  the 
third  row  from  the  dividing  line  in  either  direction;  in  other  words, 
that  it  seemed  limited  to  the  first  two  rows  on  either  side  of  that  line. 

To  test  this  conclusion  in  a  more  general  way,  the  percentages 
of  curculio  injury  to  apples  were  figured  separately  for  all  trees  in 
each  cross-row  from  which  any  fruit  was  picked,  beginning  at  the 
outer  end  of  the  sprayed  half  of  the  orchard  and  ending  at  the  oppo- 
site end  of  the  unsprayed  half.  The  details  are  shown  on  the  ac- 


1906.]  SPRAYING  APPLES  FOR  THE  PLUM-CURCULIO.  283 

companying  table  (p.  284), from  which  it  will  be  seen  that  12  trees  in 
six  different  transverse  rows  of  the  sprayed  half  of  the  orchard,  rang- 
ing from  the  first  to  the  ninth  row,  bore  from  26  to  30  per  cent,  of  in- 
jured apples, — excluding  for  the  moment  a  single  exceptional  tree 
in  the  seventh  row,  which  gave  but  16  per  cent,  of  injury.  That  is, 
with  this  exception,  the  ratios  ran  from  row  to  row  with  virtual 
uniformity  as  far  as  the  third  row  from  the  dividing  line.  In  the 
other  half  of  the  orchard^ that  left  unsprayed  as  a  check,  the  injury 
on  10  trees  distributed  through  six  rows — from  the  fourteenth  to 
the  twenty-second — ranged  from  87  to  99  per  cent.,  and  in  this  series 
also  there  is  no  uniform  progression  as  one  goes  from  the  dividing 
line  outward.  ,  ^J<bU 

We  may  make  this  influence  of  a  proximity  of  check  and  experi- 
mental plots  more  definitely  apparent  by  saying  that  if  the  two 
adjacent  rows  only  had  been  brought  into  comparison  the  difference 
in  injured  apples  would  have  been  only  38  per  cent.,  while  the  real 
effect  of  the  treatment  was  a  difference  of  63  per  cent.  Or,  to  put 
the  facts  still  more  definitely,  a  comparison  of  the  product  of  the 
adjacent  rows  would  indicate  a  saving,  .by  spraying,  of  47  per  cent, 
of  the  apples  which  would  otherwise  have  been  injured,  while  the 
real  saving  was  one  of  70  per  cent.  The  apparent  benefit  would  be 
but  two  thirds  the  actual. 

We  find,  in  short,  two  to  four  rows  running  through  the  center 
of  this  orchard,  one  or  two  in  each  of  the  two  plots,  the  product  of 
which  has  been  rendered  unfit  for  use  in  an  exact  comparison  of  the 
condition  of  these  two  plots  with  reference  to  experimental  results. 
Such  a  strip  must  evidently  always  exist  under  similar  circum- 
stances ;  a  kind  of  neutral  zone  belonging  properly  to  neither  plot, 
and  unfit  for  comparison  or  for  use  in  a  discussion  of  the  outcome  of 
an  experiment.  The  importance  of  this  conclusion  is  evident  when 
one  scans  the  published  statements  of  similar  experiments  and  finds 
that  in  many  cases  the  adjoining  plots  are  so  narrow  that  nearly 
or  quite  all  of  both  must  lie  in  this  neutral  zone.  Sometimes,  indeed, 
one  can  not  learn  from  the  description  of  an  experiment  the  form  or 
dimensions  of  the  plots,  and  consequently  can  not  judge  how  far 
some  portions  of  each  or  either  may  have  leen  removed  from  this 
central  belt.  Any  experiment  of  the  kind  on  diffusible  insects  or 
fungi  which  does  not  take  these  facts  into  account,  and  from  the  dis- 
cussion of  which  a  sufficient  central  strip  is  not  omitted,  must  be  re- 
garded as  unsatisfactory,  if  not  rejected  outright  as  inexact. 

Both  the  amount  and  the  extent  of  the  mutual  influence  of  check 
and  experimental  plots  will  vary  greatly  with  different  insects,  and 
with  the  same  insect  species  under  different  conditions  of  abundance 
and  different  kinds  of  treatment.  Where  infestation  is  unusually 
heavy  this  interference  with  effects  of  treatment  would,  other  things 
being  equal,  be  more  pronounced.  To  determine,  in  any  case,  how 


281 


BULLETIN  No.  108. 


[May, 


much  of  a  central  strip  must  be  omitted,  it  would  be  necessary  to 
make  some  such  preliminary  test  as  I  have  described,  or,  if  the  entire 
product  of  the  plots  were  harvested,  to  keep  the  different  parts  of 
it  distinct,  rejecting  those  data  which  show  an  evident  influence  of 
one  plot  upon  the  other. 

In  an  ordinary  plot  experiment,  if  I  infer  from  the  outcome  of 
the  treatment  of  a  lot  of  trees  in  the  midst  of  an  orchard  what  will 
happen  if  an  entire  isolated  orchard  were  similarly  treated,  I  tacitly 
assert  that  such  a  plot,  so  situated,  is  so  like  an  isolated  orchard  in 
all  that  concerns  my  experiment  that  the  one  may  be  substituted  for 
the  other.  As  a  matter  of  fact,  however,  while  a  small  part  of  an 
orchard  is  analogous  to  the  whole  of  it  in  many  particulars,  it  may 
differ  from  the  whole  in  just  those  particulars  and  conditions  which 
most  strongly  affect  my  problem. 

Since  reasoning  by  analogy  is  the  only  process  possible  in  such 
a  case,  we  must  first  make  this  analogy  practically  complete — must 
establish  the  essential  similarities  between  our  experimental  plot 
and  an  entire  orchard — before  we  can  reach  any  safe  conclusion  as 
to  the  practical  application  of  the  experimental  results.  This  I  have 
done  to  the  best  of  my  ability  in  the  experiment  here  referred  to, 
and  I  venture  to  think  that  like  results  may  be  expected,  under  sim- 
ilar conditions,  from  a  repetition  upon  entire  orchards  of  the  treat- 
ment here  described. 

ORCHARD  I.    SPRAYED  PLOT,  Rows  1  TO  11;  AND  CHECK,  Rows  12  TO  22. 


Row 

Trees 
examined 

Per  cent, 
injured 

Average 

1 

2 

28.95 

2 

0 

3 

1 

29.89 

4 

0 

5 

3 

29.73 

26.53 

6 

4 

28.59 

7 

1 

16. 

8 

0 

9 

1 

26. 

10 

0 

11 

3 

43.33 

43.33 

12 

3 

81.5 

81.5 

13 

0 

14 

2 

92.23 

15 

0 

16 

3 

88.08 

17 

1 

87.7 

90.33 

18 

2 

87.19 

19 

0 

20 

1 

93.79 

21 

0 

22 

1 

98.96 

1906.  ] 


SPRAYING  APPLES  FOR  THE  PLUM-CURCULIO. 


285 


DIAGRAM  OF  SPRAYING  EFFECTS  WITH  ARSENATE  OF  LEAD. 
(Showing  mutual  influence  of  check  and  experimental  plots.) 


? 

Experimental  Plot  (rows). 

Check  Plot  (rows). 

1 

• 

1234567890    11 

12    13    14    15    16    17    18    19  20    21    22 

inn  - 

on  - 

j 

"^. 

^ 

/ 

sn  - 

/ 

/ 

^"s 

••     •• 

—  — 

/ 

fin  - 

zn  - 

An  . 

/ 

30- 

-____; 

^=" 

•^M-S 

_____ 

/ 

/ 

, 

/ 

— 

\ 

7 

in 

\ 

/ 

n 

The  above  diagram  is  simply  a  graphic  representation  of  the  data 
of  the  preceding  table.  The  vertical  lines  represent  the  rows  of  trees 
in  Orchard  i,  except  the  central  line,  which  marks  the  division  be- 
tween the  half  of  the  orchard  which  was  sprayed  and  the  half  left 
unsprayed  as  a  check.  The  horizontal  lines,  numbered  from  10  to 
100,  indicate  percentages  of  injured  apples.  The  broken  line  cross- 
ing the  diagram  from  left  to  right  intersects  the  vertical  line  for 
each  row  at  a  point  corresponding  to  the  percentage  of  injured  apples 
from  that  row.  It  will  be  noticed  that  the  eleventh  row  of  the  series 
is  the  experimental  row  next  to  the  check  plot,  and  that  the  twelfth 
is  the  check  row  next  to  the  experimental  plot.  The  influence  of 
these  plots  one  upon  the  other  is  shown  by  the  strong  upward  course 


286  BULLETIN  No.  108.  [M<ty- 

of  the  broken  line  from  the  ninth  to  the  eleventh  rows,  and  the  down- 
ward course  from  the  fourteenth  to  the  twelfth.  The  short  horizon- 
tal lines,  one  on  each  side  of  the  diagram,  indicate  the  average  per- 
centage of  injury  on  check  and  experimental  plots  respectively, 
with  the  exception  of  rows  eleven  and  twelve.  The  distance  between 
the  inner  ends  of  these  lines  is  a  measure  of  the  benefit  resulting 
from  the  spray,  and  the  distance  from  the  inner  end  of  each  line  to 
the  point  where  the  broken  line  crosses  the  vertical,  is  a  measure  of 
the  influence  of  the  other  plot  through  the  unequal  interchange  of 
curculios. 


3  ! 


